1. Field of the Invention
The invention relates to varistors and more specifically to varistors having high energy absorption.
2. Summary of the Prior Art
Varistors having high energy absorption capabilities are difficult to fabricate. This difficulty is due to the complex chemical nature of the varistor mixture. Varistors are mainly composed of zinc oxide in combination with selected quantities of Bi.sub.2 O.sub.3, Sb.sub.2 O.sub.3, Co.sub.3 O.sub.4, MnO.sub.2, SiO.sub.2 and smaller levels of B, K, or Na, and Al.sub.2 O.sub.3. Energy is absorbed in the varistor by Joule heating with the ZnO grains acting as heat sinks.
Increasing the grain size to increase energy absorption through traditional ceramic procedures of extending sintering times or higher sintering temperatures is possible, but other electrical properties are adversely affected. Additionally, the varistor may lose portions of the key constituents due to their volatile nature at high sintering temperature. Components of the mixture that are lost at higher sintering temperatures include Sb.sub.2 O.sub.3, B, K and, in particular, Bi.sub.2 O.sub.3. Loss of these materials results in an increase in porosity of the disc, causing the maximum energy absorption to be reduced. Stated alternatively, a particular time/temperature combination produces maximum absorption, with a decrease in absorption occurring with either higher sintering temperature or reduced processing time.
In order to overcome the detrimental effects of extending sintering time, a more refractory chemical composition was desired which would tolerate the higher processing temperatures required to grow the larger ZnO grains. In addition, the chemical composition needed to be such that the other electrical parameters of the varistor were not degraded. The disclosed invention meets these requirements.